1. Field of The Invention
The present invention generally relates to a flash fixing color toner that is used with an electrophotographic image forming process which is used by an electrophotographic copier, facsimile or printer, and more particularly to a flash fixing color toner which is appropriately fixed onto a recording medium by irradiation with flash light in an electrophotographic image forming process.
2. Description of The Related Art
In an electrophotographic image forming apparatus that employs flash light irradiation for fixing a toner onto a recording medium, the application of pressure to the toner is not needed for the fixing of the toner but it is necessary to make efficient use of the energy of flash light in the fixing or fusing of the toner onto the recording medium.
Generally, a color toner absorbs only part of an irradiation light energy, apart from a black toner. It has been observed that the light energy absorption properties of a color toner are lower than the light energy absorption properties of a black toner, and that the use of a color toner in a conventional flash fixation process often causes a defective fixing. However, in recent years, there is an increasing demand for a color image formation using an electrophotographic flash fixation process, which is capable of performing a high-speed operation and usable with a special recording medium having a step-like surface.
There are basically five major steps employed in the electrophotographic printing process: (1) charging a photoconductor electrostatically, (2) exposing the photoconductor to the imaging light pattern to create an electrostatic latent image thereon, (3) developing the photoconductor by bringing charged toner particles to the surface of the photoconductor to create a toner image thereon, (4) transferring the toner image from the photoconductor surface to a recording medium (e.g., paper), and (5) fixing or fusing the toner to the recording medium.
As is well known, among these steps of the electrophotographic printing process, the toner fixing step may be achieved by selecting one of the three methods: the heat roll method, the oven fixing method and the flash fixing method. The flash fixing method uses irradiation of light or infrared rays (flash light).
The heat roll method mentioned above is most commonly utilized. In the heat roll method, the fixing roller held at a high temperature applies heat and pressure directly to the toner so that the toner is fixed to the recording medium. With a simple, inexpensive configuration of the fixing roller used, the fixing surface can be made flat. However, there are several problems with the heat roll method. The recording medium (e.g., paper) after the toner fixing step is liable to be curled. The toner sticks to the surface of the fixing roller, and the recording medium is liable to being stained with such toner due to the offset. The recording medium and the fixing roller contact each other, and the transport of the recording medium in an image forming apparatus may be slanted from the desired direction. It is difficult to achieve high-speed image formation with the image forming apparatus. It is difficult that the heat roll method achieves the toner fixing of a special recording medium like a sticker post card.
The flash fixing method mentioned above is a non-contact toner fixing method, and the problems of the heat roller method such as the curling and the offset are eliminated. The high-speed image formation and the toner fixing of a special recording medium can easily be achieved with the flash fixing method. Accordingly, the application of the flash fixing method to high-speed printers and copiers for office use are increasing in recent years.
In the flash fixing method, a black toner or the like efficiently absorbs an irradiation light energy with respect to all the wavelengths. A thermal energy produced by irradiation of flash light will easily increase the temperature of the black toner to about 200 deg. C., and the particles of the black toner can be fused without difficulty. The toner fixing to the recording medium can easily be achieved in the case of the black toner or the like. However, as described above, the light energy absorption properties of a color toner are lower than the light energy absorption properties of a black toner, and the use of a color toner in the conventional flash fixation process often causes a defective fixing. For example, a thermal energy produced by irradiation of flash light can increase the temperature of the color toner to 100 deg. C. at the highest, and the particles of the color toner cannot easily be fused. The toner fixing to the recording medium cannot easily be achieved in the case of the color toner. The use of a color toner in the conventional flash fixation process is likely to cause a defective fixing.
As disclosed in Japanese Patent Nos.1602417 and 1820258, the inclusion of an infrared absorbent in a flash fixing color toner has been proposed in order eliminate the above problem. However, the infrared absorbent generally is expensive. As disclosed in Japanese Patent No.3022110, the infrared absorbent is a coloring agent which affects the resulting color image, and the use of the infrared absorbent imposes restrictions in many cases. Hence, it has been observed that it is desirable to reduce the amount of an infrared absorbent in a flash fixing color toner as small as possible.
Japanese Laid-Open Patent Application No.9-179347 discloses the inclusion of a small amount of carbon in a flash fixing color toner. Japanese Laid-Open Patent Application No.9-329912 discloses the inclusion of a small amount of coloring dyes or pigments in a flash fixing color toner. These measures are taken in the above documents to convert the color toner into a substantially black toner by inclusion of the secondary substances in order to improve the light energy absorption properties of such color toners and increase the efficiency of the color toner fixation to the recording medium.
Although the efficiency of the toner fixation in the cases of the above documents is increased to a certain extent, the substantially black toner, derived by the inclusion of the secondary substances, is liable to affecting the resulting color image, and the quality of the resulting color image, in particular, the sharpness, is low.
It is an object of the present invention to provide an improved flash fixing color toner in which the above-described problems are eliminated.
Another object of the present invention is to provide a flash fixing color toner that increases both the efficiency of the toner fixation and the sharpness of a resulting color image by using a hue-altering coloring agent having such characteristics that adequate light energy absorption properties are retained with the initial hue of the color toner prior to the irradiation of flash light and that the initial hue of the color toner is altered at the time of the irradiation of flash light into the secondary hue, the secondary hue providing adequate sharpness of the resulting color image.
Another object of the present invention is to provide an image forming process that increases both the efficiency of the toner fixation and the sharpness of a resulting color image by using a flash fixing color toner containing a hue-altering coloring agent having such characteristics.
The above-mentioned objects of the present invention are achieved by a flash fixing color toner for use in an image forming process, the color toner comprising a binder resin and a hue-altering coloring agent, wherein the hue-altering coloring agent alters, at a time of irradiation of flash light, an initial hue of the color toner into a secondary hue, so that the color toner is fixed to a recording medium by the irradiation of flash light, and a color image is substantially formed with the secondary hue of the color toner.
The above-mentioned objects of the present invention are achieved by an image forming process, which is characterized by using a flash fixing color toner, the color toner comprising a binder resin and a hue-altering coloring agent, the image forming process comprising the steps of: producing a toner image on a photoconductor by using the color toner; transferring the toner image from the photoconductor to a recording medium; and fixing the color toner to the recording medium by irradiation of flash light, so that a color image is formed on the recording medium, wherein the hue-altering coloring agent alters, at a time of the irradiation of flash light, an initial hue of the color toner into a secondary hue, so that the color toner is fixed to the recording medium by the irradiation of flash light, and the color image is substantially formed with the secondary hue of the color toner.
In the flash fixing color toner of the present invention, the hue-altering coloring agent alters the initial hue of the color toner at the time of irradiation of flash light into the secondary hue of the color toner after the toner is fixed to a recording medium by the irradiation of flash light. According to one preferred embodiment of the color toner of the present invention, the hue-altering coloring agent has the initial hue of the color toner prior to the irradiation of flash light, the initial hue being black, dark brown, brown or dark violet and providing adequate light energy absorbing properties. The hue-altering coloring agent has the secondary hue of the color toner after the toner is fixed to the recording medium by the irradiation of flash light, the secondary hue being yellow, reddish yellow, red, blue or green and providing adequate sharpness of a resulting color image to avoid the deterioration of the hue of the color image.
Accordingly, the flash fixing color toner of the present invention using the hue-altering coloring agent is effective in increasing both the efficiency of the toner fixation and the sharpness of the resulting color image.
Other objects, features and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawing.